It has been expected that calcia would be useful as a refractory material for a steel-making furnace, a ladle and a cement-kiln, because calcia exhibits an extremely high melting point of about 2580.degree. C., an excellent resistance to basic slag and superior dephosphorizing, desulfurizing and deoxidizing properties. However, calcia has disadvantages in that, when placed in an ambient atmosphere, a conventional type of calcia absorbs the moisture in the ambient atmosphere and is pulverized due to the reaction shown below and, finally, is decomposed. EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2
That is, conventional calcia exhibits an extremely poor resistance to slaking. The slaking phenomenon causes the mechanical strength of the calcia to decrease. Therefore, it is difficult to utilize conventional calcia in a practical industrial use and it is difficult to store it without the quality being degraded.
Various attempts as indicated below, were made to eliminate the above-mentioned disadvantages from the calcia.
(1) A method in which a lime material is melted at a temperature higher than the melting point of CaO.
(2) A method in which a lime material is sintered at an elevated temperature.
(3) A method in which an additive is admixed with a lime material.
In method (1), a calcia is mixed with a small amount of SiO.sub.2 and/or Fe.sub.2 O.sub.3, the mixture is melted at a temperature of 3000.degree. C. or more for several hours and the melt is used for producing a desired refractory material. This method is disclosed in Japanese Patent Application Publication No. 50-27203 (1975).
In the method (2) which is disclosed by Japanese Patent Application Laid-open No. 52-53908 (1977), a calcia is heated at a temperature of 1600.degree. C. or more, but not exceeding the melting point of the calcia, for 2 hours or more, and the heated calcia is cooled at a cooling rate of 100.degree. C./hr or less to provide a particulate calcia clinker in which the calcia crystals have a diameter of 5 microns or more.
In method (3) which is disclosed in Japanese Patent Application Laid-open Nos. 53-55311 (1978) or 50-14709 (1975), a calcia is mixed with 0.4 to 1.3 molar % of Fe.sub.2 O.sub.3 or a mixture of a calcia with 2 to 10% by weight of Fe.sub.2 O.sub.3 and TiO.sub.2 is sintered.
However, in method (1), the calcia is heated at an extremely high temperature, higher than the melting point of the calcia, with a large consumption of heat energy. Therefore, this method (1) is high in cost and is not adequate for mass production of the calcia clinker. Also, the resultant calcia clinker exhibits an unsatisfactory resistance to slaking.
In method (2), the burning procedure is carried out at an elevated temperature of 1600.degree. C. or more over a long time period with a great consumption of heat energy. The cooling procedure must be carried out at a limited cooling rate to produce a calcia clinker having a good quality. Therefore, a usual rotary kiln is not adequate for the calcining and cooling procedures. Also, the quality of the resultant calcia clinker is still unsatisfactory.
In method (3), the burning temperature for a lime material must be carried out at a very high temperature and the resultant calcia clinker containing Fe.sub.2 O.sub.3, etc. and/or TiO.sub.2 exhibits an unsatisfactory modulus of rupture and resistance to corrosion.